“Silica” deposits are not pure silica but a complex mixture of polymeric silica, calcium silicate, magnesium silicate, calcium carbonate, and smaller amounts of other inorganic compounds. In most of the aqueous systems, where there is a need to inhibit silica or silicate scale, there is also a need for the simultaneous inhibition of the calcium carbonate scale, and the need to disperse silt and the bulk water precipitation of silica, silicates and carbonates.
Some of the most difficult deposits encountered in cooling, reverse osmosis, mining and geothermal water systems are those comprised of silica. The temperature and pH of the water affects silica precipitation and deposit formation. The pH of geothermal brines is generally 4.0 to 6.0 and the brine temperature is generally about 100° C. to 210° C. The temperature of cooling water is generally about 30° C. to 80° C. and the pH is generally about 6.0 to 9.0. Cooling water is also exposed to cathodic microenvironments within corrosion cells on the metallic heat transfer surfaces where the pH is about 9.0 to 9.5 and higher.
Several methods are used to prevent or inhibit silica deposits. The simplest method involves keeping silica, calcium and magnesium below the critical concentration levels necessary for the precipitation of silicates. The critical concentrations suggested by the water treatment industry are: (1) at pH<7.5, silica (as SiO2)<200 ppm; and (2) at pH>7.5, silica (as SiO2)<100 ppm. If magnesium is present then (expressing Mg as ppm CaCO3 and Si as ppm SiO2), the recommended concentrations are (1) at pH<7.5 (Mg×Si) is <40,000, and (2) at pH>7.5 (Mg×Si) is <20,000.
Various chemical treatment methods have been developed that inhibit silica/silicate and other scale/deposits. Anionic polymers, cationic polymers, organic phosphonates, boric acid, and its sodium salts are described in the patent literature. Of particular interest is U.S. Pat. No. 5,078,879 which discloses a method for controlling silica/silicate deposition in a aqueous system with an admixture of 2-phosphonobutane-1,2,4-tricarboxylic acid and a water-soluble polymers based on acrylic and sulfonic monomers, but not on the allyloxybenzenesulfonate monomer..
Scale inhibitors are needed which will inhibit multiple scales very efficiently in an economical way. Currently the most effective silica/silicate anti-scalant is an acrylic terpolymer from Rohm&Haas sold under the trade name ACUMER® 5000. ACUMER 5000 is disclosed in U.S. Pat. No. 5,277,823. The exact composition of ACUMER 5000 is not disclosed, but the above mentioned patent gives strong indication that the polymer consists of selection of monomers such as AMPS, acrylic, maleic and others but not allyloxybenzenesulfonate monomer. This low molecular weight, water-soluble polymer is an excellent anti-scalant, but it is expensive. Thus there is a need for anti-scalants that have equal or improved effectiveness, but which are less expensive than ACUMER 5000.
U.S. Pat. No. 4,915,845 discloses the use of water-soluble polymers of allyloxybenzenesulfonate monomer and their use in aqueous systems in dispersing particulate matter, particularly drilling mud. Although the above mentioned polymer is also claimed as an inhibitor for the mineral scale in an aqueous systems but not specifically for the inhibition of silica and silicates and not in an admixture with other inhibitors.